The invention relates to a plasma melting furnace comprising a water-cooled bottom electrode of copper, a temperature probe connected with the bottom electrode, and a wearing part of steel covering the bottom electrode in the bottom of the furnace, at least one counter electrode for the formation of the plasma jet being arranged at a distance above the wearing part.
With a plasma melting furnace of this kind the plasma jet is led between the bottom electrode (anode) and the counter electrode(s) (cathode(s)). The water-cooled bottom electrode is supervised by a temperature measuring device, which means that the electrodes are switched off when exceeding a certain temperature in order to prevent a breakthrough of water into the steel bath of the furnace.
During a furnace campaign the refractory lining of the furnace gets worn, the wearing part at the bottom electrode melting off accordingly and shortening in the direction of the water-cooled bottom electrode. In case of a plurality of counter electrodes, the bottom electrode provides for the current of all plasma burners.
With the usual technical sizes of known plasma furnaces, the summation current of the bottom electrode amounts to between 10,000 and 50,000 A. What is decisive to the faultless functioning of the furnace is a good contact of the scrap or bath with the wearing part at the bottom electrode. In case of an insufficient electrical conductivity of the contact site in the region of the bottom electrode, secondary arcs may form between the scrap and the wearing part.
Towards the end of a furnace campaign it may furthermore happen that the refractory lining gets damaged in the immediate vicinity of the bottom electrode when the scrap sets. This may also lead to the formation of a secondary arc at the bottom electrode between a piece of scrap and the wearing part.
Secondary arcs of this kind may lead to a strong local overhearing of the wearing part and of the bottom electrode itself, thus creating the danger of a melting through of the entire bottom electrode (in the manner of a torch cut) as far as into the water-cooled section. In case of such a breakthrough, the cooling water, which is under pressure, would penetrate into the furnace below the molten bath and would lead to oxyhydrogen gas explosions, constituting a risk to the furnace and to the operating personnel. The process of melting through of the electrode takes place at a very high speed so that the temperature measuring means will not be able to give a warning signal in order to shut down the plant.